Powder X-ray diffraction combined with solid state and 129Xe NMR to study supramolecular crystalline materials

The study of crystalline architectures showing permanent porosity that can absorb molecules from the gas phase or include selectivelytarget polymer segments in the nanochannels will be addressed [1]. The porous crystalline matrices span from dipeptide crystals and molecular zeolites to metal-organic frameworks and crystalline hybrid organosilicas [2], [3]. The open pore structure of the nanochannels and the diffusion rates of the gases was established by hyperpolarized Xenon NMR whilst gases such as carbon dioxide and methane were observed directly by 1H and 13C NMR spectroscopy after gas diffusion into the cavities. We recognized, for the first time, the presence of parallel and aligned molecular rotors in the crystalline pore walls of nanoporous hybrid materials, yielding an unsual anisotropic arrangement of surfaceexposed molecular rotors in a 3D framework [4]. The entrapment of molecules inside the channels could successfully regulate the fast molecular rotor dynamics. The spontaneous formation of the supramolecular architectures were realized by a solvent-free mechanochemical approach or by thermal treatment of a host molecule and bock copolymers, as demonstrated by in situ synchrotron X-ray diffraction. The driving force for the fabrication of crystalline inclusion compounds with target segments of copolymers was based on the establishment of cooperative noncovalent intermolecular interactions, while steric effects prevented the formation of the inclusion-crystals with the remaining blocks. The 2D 1H-13C solid state and fast-1H MAS NMR provided direct evidence of the intimate interactions between the host and selected block. The large magnetic susceptibility generated by the aromatic host groups surrounding the included segments could be described by ab initio calculations that enabled the evaluation of short intermolecular distances between the host and the target block, demonstrating the existence of a diffuse network of multiple CH··· π host-guest interactions. The partitioning of the copolymer blocks in inclusion crystals regularly alternated by amorphous nanophases could fabricate architectures on molecular and nanometric hierarchical scales, as demonstrated by high resolution SEM images. By a similar strategy, a flexible homopolymer was transformed into a semicrystalline polymer through the partial inclusion of chainsegments within the cavities of a crystalline host [5]. The self-assembly process was realized by simple co-grinding of minor amounts of host molecules and the polymer bulk. This assembly led to the formation of innovative nanocrystalline adducts which behaved as cross-linking nanodomains. A multitechnique approach comprising powder XRD and fast-1H solid state NMR enabled the identification of the unusual phase architecture in the material.